The present invention relates to a process for the production of bismuth germanate monocrystals with a high scintillation response.
It more particularly applies to the field of producing scintillators used as high energy radiation detectors in medicine (tomoscanners, positron cameras), in high density physics and in oil and mining research.
Certain of these applications require large crystals (diameter&gt;50 mm, length&gt;200 mm) and in all cases an optimum luminous efficiency is required.
It is therefore necessary to avoid impurities, which can reduce the light emission intensity, as well as anything which is liable to reabsorb it during its path within the material (impurities, precipitates, various inclusions, etc).
The most widely used method for producing monocrystals of bismuth germanate (currently called BGO) is the Czochralski method. This is a pulling or drawing method consisting of raising to the boiling point (T.sub.F =1050.degree. C.) in a suitable crucible a polycrystalline charge close to the desired composition. A small monocrystalline bar of an appropriate size and orientation, called a germ, is then brought into contact with this molten bath. As from this time, a slow translation of this germ (a few mm/h) as well as a control of the crucible temperature permit the growth of a monocrystal having the desired geometry, provided that there is a suitable thermal environment and a satisfactory atmosphere.
At present, BGO monocrystals are produced using platinum crucibles and an oxidizing atmosphere.
Such a prior art BGO monocrystal production process is described in the article by O. H. NESTOR and C. Y. Huang (Harshaw Company) in IEEE Transactions on Nuclear Science, V45, 22, February 1975. This process makes it possible to obtain large monocrystals having scintillation properties. However, all these crystals have layers of macroscopic defects (blisters, inclusions, filaments, etc) which diffuse light, reduce the overall scintillation response and prejudice the homogeneity of the light response along the ingot.
Metallurgists are aware of the fact that the alloy Pt-Bi forms at 730.degree. C. The melting point of BGO is 1050.degree. C. making it necessary to use platinum crucibles under an oxidizing atmosphere to prevent the presence of bismuth metal. In spite of this, there is a corrosion of the crucible with accumulation of metallic impurities from the platinum within blister-type inclusions of the crystal.
Numerous scientific publications describe all the defects of monocrystals produced according to the said prior art process and the way in which the numbers therof can be reduced (for example K. TAKAGI, T. FUKAZAWA, M. ISHII, S. AKIYAMA (Hitachi), J. of Crystal Growth, 52, (1981), pp. 584-587).
These methods for improving the quality of BGO crystals, such as the use of pure starting substances, the growth of the crystals at a slow speed, successive crystallizations, large rotation of the crystal, etc are onerous and take a long time to perform, whilst only having a moderate effectiveness.